Basics of photometry

Photodiodes

Basics of photometry

This is a brief introduction to the basics of photometry.

To be able to understand this subject better a brief review of geometric principles utilised is required.

Geometric principles

Radian

In plane geometry the angle whose arc is equal to the radius generating it is called a radian. Therefore, if C = 2¹R (Circumference of a circle) 2¹R = 360°. Radian = 180°/¹ = 57.27° (approx.).

Figure 1

Other abbreviations used.

Ae = Area of emitting (or reflecting) surface

Ap = Apparent area of an emitting source whose image is projected in space and viewed at some angle, q.

Ad = Detection area. Whether a physical target or merely a defined spatial area, it is the area of interest.

Steradian

In solid geometry one steradian is the solid angle subtended at the centre of a sphere by a portion of the surface area equal to the square of the radius of the sphere. Therefore, if AREA/R² = 1 = 1 steradian and the area on the surface of a sphere equals 4¹R², then 4¹R²/R² or 4¹ steradians of solid angle w about the centre of a sphere. The steradian is usually abbreviated as STER.

Figure 2

Photometric Terminology

Flux (Symbol F)

Any radiation, whether visible or otherwise, can be expressed by a number of FLUX LINES about the source, the number being proportional to the intensity of that source. This LUMINOUS flux is expressed in LUMENS for visible radiation.

Luminous emittance (Symbol L)

A source measurement parameter. It is defined as the ratio of the luminous flex emitted from a source to the area of that source, or L = F/Ae. Typically expressed in units of:

lumens/cm²or one PHOT,

lumens/m²or one LUX (or one METRE CANDLE), lumens/ft²or one FOOT CANDLE.

Illuminance (Symbol E)

This is a target or detector area measurement paragraph meter. It is the ratio of flux lines incident on a surface to the area of that surface or E = L/Ad. Typical measurement units are the same for LUMINOUS EMITTANCE ie. lumen/cm² = one phot, lumen/m² = one lux, and lumen/ft²= one ft candle.

Figure 3

Three dimensional figure

Two dimensional figure

Data Sheet

Luminous intensity (Symbol I)

A spatial flux density concept. It is the ratio of luminous flux of a source to the solid angle subtended by the detected area and that source. The LUMINOUS INTENSITY of a source assumes that source to be point rather than an area dimension. The LUMINOUS INTENSITY (or CANDLE POWER) of a source is measured in LUMENS/STERADIAN which is equal to one CANDELE (or loosely, one CANDLA).

Figure 4

Luminance (Symbol B)

Sometimes called photometric brightness (although the term brightness should not be used alone as it encompasses other physiological factors such as colour, sparkle, texture, etc.) it is applied to sources of appreci-able area size. Mathematically, if the area of an emitter (circular for example) has a diameter or diagonal dimension greater than 0.1 the distance to the detector, it can be considered as an area source. If less than this 10% figure, the source can be treated as point in nature. This one to ten ratio of source diameter to distance is offered as it MATHEMATICALLY very closely approximates results obtained when comparing an area source to its point equivalent. LUMINANCE presents itself as an extremely useful parameter as it applies a figure of merit to:

1. Apparent or protected area of the source (Ap).

2. Amount of luminous flux contained within the projected area of the source (Ap).

3. Solid angle the projected area generates with respect to the centre of the source.

Note. The projected area Ap varies directly as the cosine of q ie. max. at 0° or normal to the surface and minimum at 90°

Ap = Ae cos q

LUMINANCE is defined as the ratio of LUMINOUS INTENSITY to the projected area of the source Ap.

Figure 5

LUMENS

LUMINOUS INTENSITY = STERADIAN = CANDELAS Ap Ae cos q (Sq. Unit) And depending on the units used for area:

1 CANDELA/cm² = 1 STILB 1 CANDELA/m² = 1 NIT

1 CANDELA/in² = no designator available.

1 CANDELA/ft² = Also:

l/¹ candela/cm² = LAMBERT

l/¹ candela/m² = APOSTILB (or BLONDEL) l/¹ candela/in² = no designator available l/¹ candela/ft² = FOOT LAMBERT

CIE curve

Photometric quantities are related to the corresponding radiometric quantities by the CIE Standard Luminosity Function which is often called the 'standard eyeball'.

The eye responds to the rate at which radiant energy falls on the retina, ie., on the radiant flux density expressed as Watts/m². The corresponding photometric quantity is Lumens/m². The standard luminosity function is then, a plot of Lumens/Watt as a function of wavelength.

The function has a maximum value of 680 Lumens/Watt at 555nm and the ¹/2power points occur at 510 nm and 610 nm (Figure 6).

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Figure 6 CIE stand photopic luminosity function

Wavelength (nm)

Electromagnetic radiation spectrum

Figure 7 Electromagnetic spectrum

Figure 8 Typical quantum efficiency curves

The following range of discrete opto devices is described, each of which may be used in a variety of sensing applications.

Product RS stock no.

General purpose photodiode 305-462 BPX 65 high speed photodiode 304-346

BPW21 photodiode 303-719

Quadrant silicon photodiode 652-027 15mm²silicon photodiode 194-076 Medium area photodiode 651-995 Large area photodiode 303-674 Integral amplifier 5mm² 308-067 Integral amplifier 100mm² 590-963 5.8mm²UV photodiode 564-021 33.6mm²UV photodiode 564-037 100mm²UV photodiode 564-043 16 element linear array 194-060

General purpose photodiode

(RS stock no. 305-462)

A planar diffused photodiode in a 2-lead TO-18 can with glass window. A very linear output of current versus light level can be obtained over a wide range of inputs.

Light falling on the diode induces current in the diode, thus when the device is reversed biased thereby conducting very low leakage currents, it behaves as a current source controlled by the incident illumination.

Absolute maximum ratings

at +25°C (unless stated)

Reverse voltage V_R _________________________+80V Forward current I_F ________________________100mA Operating temperature range _________0°C to +70°C Storage temperature range_________-55°C to +125°C Power dissipation Pd ______________________200mW

Pin connections and case dimensions

Case is connected to Pin 2.

Chip placement accuracy ±0.25mm of can centre.

Nominal photosensitive area 850 mils (near square)*.

*Note: 850 mils 0.7mm ^W ´ 0.7mm

Electrical characteristics

Symbol Parameter Min. Typ. Max. Units Test conditions

V_(BR) Breakdown voltage 80 V Dark; rev. current 10µA

I_D Dark current 1.4 14 nA Dark; rev. bias 20V

R_e Responsivity 0.35 0.7 1.4 µA/mW/cm² Zero bias; 400µW/cm²

C Capacitance 12 pF Dark; rev. bias 10V

t_R Response time 4 ns 10-90% levels

- Temp. coeff. of responsivity 0.35 % per °C 0°C to +70°C

- Temp. coeff. of dark current ´2 per 10°C rise 0°C to +70°C

Figure 9 Photo current vs. irradiation

Figure 10 Open circuit voltage vs. irradiation

Typical performance curves

Irradiation - mW/cm²

Irradiation - mW/cm²

Figure 11 Normalised capacitance vs. bias voltage

Figure 12 Photo current vs. bias voltage

Figure 13 Normalised spectral response

Typical applications

Analogue light level sensor - direct reading

Low input bias current op amps such as LM 308 or FET input types can be used to give steady dc indication of light levels as is necessary for photometric applications, photocell measurements, transmission and reflection coefficients, etc.

The values shown give approximately 14V/mW/cm²of irradiation. The value of R1 and R2 may be reduced for less sensitivity but should be kept equal. For values less than 100k½, a less sophisticated amplifier may be used, eg. µA741.

The 1000pF capacitors may be increased to reduce ripple from ac lighting or control response time accor- dingly.

Figure 14 Analogue light level sensor - direct reading

ac coupled analogue sensor for optical communications links

A stage of amplification giving the substantial gain necessary for optical communication links is im- plemented as shown. An op amp with low input bias currents such as LM308 or an FET input type is necessary.

The input ac coupling C1 gives a dc isolation of steady ambient conditions, and C2 minimises effects of offset voltages, both such lower break frequencies are below 10Hz.

Upper frequency response is approximately 3kHz and ac sensitivity is 70V/mW/cm². A further amplifier/buffer stage is necessary to drive a headset or loudspeaker.

Bias voltage - V

Bias voltage - V

Wavelength - µm

Figure 15 ac coupled analogue sensor for optical communications links

BPX 65 high speed photodiode

(RS stock no. 304-346)

The BPX 65 is a planar silicon PIN photodiode housed in a modified TO-18 case incorporating a plain glass flat window which has no influence on the beam path of optical lens systems. The cathode is electrically connected to the case. Because the BPX 65 is capable of detecting wide bandwidth signals due to its excellent high frequency response, this coupled with its high sensitivity makes the device ideal for signal detection applications. This photodiode is outstanding for low junction capacitance and short switching times.

Absolute maximum ratings

at +25°C (unless stated)

Reverse voltage V_R___________________________50V Forward current I_F ________10mA (200mA pulsed 1µs 1% duty cycle) Operating temperature range _______-25°C to +70°C Storage temperature range_________-55°C to +125°C Junction temp. T_J__________________________+125°C Power dissipation Pd ______________________250mW

(derate linearly 2.5mW/°C above +25°C) Logarithmic detector for exposure meter

Feeding the RS stock no. 305-462 into a high impedance gives a logarithmic voltage/illumination response.

The circuit shown is the basis for a simple battery- operated exposure meter. At very low light levels where the amplifier bias current may cause the output to go negative, a diode avoids the spurious state of negative indication.

The movement may be calibrated in photographic scales, one stop being approximately 7µA. Sensitivity can be trimmed by adjusting R1.

Figure 16 Logarithmic detector for exposure meter

Shape and dimensions

Electrical characteristics at +25°C

Parameter Test conditions min. typ. max. units

A Radiant sensitive area 1 mm²

ls max. Wavelength of max. sensitivity 850 nm

R_e Responsivity l =450nm 0.2 A/W

l =900nm 0.55 A/W

l =1064nm 0.15 A/W

t_r Response time (10-90% levels) R_L=50½; VR=20V; l =900nm 0.5 1 ns

C₀ Capacitance V_R=0V 15 pF

C₁ V_R=1V 12 pF

C₂₀ V_R=20V 3.5 pF

f_g Cut-off frequency 500 MHz

1_p Dark current V_R=20V, Dark (E=0) 1 5 nA

S Spectral sensitivity V_R=20V; see Note 1 7 10 nA/Lx

NEP Noise equivalent power V_R=20V 3.6´10^-14 W/Ã^Hz

Note1. The illuminance indicated refers to unfiltered radiation of a tungsten filament lamp at a colour temperature of 2856K (standard light A in accordance with DIN 5033 and IEC publ. 306-1).

Typical performance curves

Figure 17 Normalised spectral responses (l) and quantum yield h (l)

Figure 18 Variation of output current with illumination

Figure 20 Variation of diode dark current with reverse voltage

Figure 19 Polar sensitivity curve

Figure 21 Variation of output current with temperature

Figure 22 Variation of diode capacitance with reverse voltage

Figure 24 Variation of dark current with diode temperature

Figure 23 Permissible power dissipation

BPW21 photodiode

(RS stock no. 303-719)

A silicon photodiode housed in an hermetically sealed case with a flat window incorporating built-in colour correction. Sensitivity approximating the human eye response. Linear current (short circuit) versus illumination. Log. voltage versus illumination.

This photodiode is designed for use in the photoamperic mode and is ideally suited for use in light monitoring and control, optical instrumentation and camera control.

Absolute maximum ratings

Ambient temperature range ________-25°C to +100°C Reverse voltage, V_R__________________________10V Open circuit voltage _______________________650mV Power dissipation (at 25°C), Pd _____________250mW Illuminance, E _____________________________10⁵lux

Features

● Response approximating to the human eye

● Photovoltaic cell operation

● Linear output current versus illumination

● Hermetically sealed TO5 case.

(For standard illuminant A, 1000 Lux = 4.75mW/cm²) Figure 25 Photovoltage vs illumination

Electrical characteristics

Parameter Conditions Min. Typ. Max. Unit

S_k Sensitivity (short circuit) R_L= 100½. EA= 10^-2to

10⁵lux* 4.5 7 nA/Lux

V_ph Photovoltage (open circuit) E_A= 1 Lux* 250 350 mV

T_kI_SC Temp. Coeff. of short circuit current E_A= 1K Lux* -0.05 %/°C

T_kV_ph Temp. Coeff. of open circuit voltage R_L= 100½. EA= 1K Lux* -2 mV/°C

lp Peak wavelength sensitivity 560 nm

Spectral bandwidth 50% sensitivity upper limit 680 nm

50% sensitivity lower limit 440 nm

Junction capacitance V_R= 0V 490 pF

t_r Rise time R_L= 1K½. VR= 5V 1.0 µs

I_D Dark current R_L= 1K½. VR= 5V 2 30 nA

NEP Noise equivalent power V_R= 5V 1.4 ´ 10^-5 Lux/ÃHz

*The illuminance indicated refers to unfiltered radiation of a tungsten filament lamp at a colour temperature of 2856°K (standard Light A in accordance with DIN 5033 and IEC publ. 306-1).

Figure 26 BPW21 and human eye spectral sensitivity

ACTIVE AREA 7.5mm2 FIELD OF VIEW 124¡ MAX

Figure 27 Photocurrent vs illumination

Quadrant silicon photodiode

(RS stock no. 652-027)

A silicon photodiode containing four separate sensing elements (with commoned cathodes) arranged one per quadrant. The output voltage of each quadrant is available separately enabling null conditions to be detected with equal degrees of shading. The device is hermetically sealed in a TO5 package which incorporates the pcb pin connections.

Absolute maximum ratings

dc reverse voltage ___________________________15V Peak pulse current (1µs, 1% duty cycle) ______200mA Peak dc current ____________________________10mA Storage temperature range_________-45°C to +100°C Operating temperature range _______-25°C to +75°C Lead temperature soldering (5s)____________+200°C

Features

● High blue sensitivity and shunt resistance

● Suitable for low light level applications

● T05 package incorporating pcb pin connections.

Applications

● High accuracy position sensing

● Alignment

● Optical surveying.

Typical photovoltaic connection

Figure 28

V₀= R_f, I_SC ISC = E^V, S_k V₀= R_f, E_V, S_k where

V₀= Signal output voltage, V R_f= Feedback resistance, ½

I_SC= Photodiode short circuit current, A E_V= Incident illumination, Lux S_k= Photodiode sensitivity, A/Lux

Pin connections and dimensions

NOTE:

Specification

Parameter Conditions Min. Typ. Max. Unit

Operating voltage 12 V

Dark current V_R= 1V 0.03 3 nA

Capacitance V_R= 0V 80 100 pF

Responsivity 900nm, V_R= 1V 0.42 0.45 A/W

Rise time 0-70%, 864nm, V_R= 10V, 100½ load <15 ns

Peak wavelength 820 nm

Spectral response range 430 900 nm

Noise equivalent power 900nm 1 ´ 10^-13 WH-^1/2

Active diameter 3 mm

Total active area 7 mm²

Metallurgical separation 200 µm

Figure 29 Typical spectral response

15mm

silicon photodiode

(RS stock no. 194-076)

A 15mm²silicon photodiode housed in an hermetically sealed TO5 package. This device is ideal for low light level applications where a very high signal to noise ratio is important such as light monitoring and control applications.

It may be operated photovoltaically or with a reverse bias of up to 12V where lower capacitance is needed.

Electrical/Optical specifications

Characteristics measured at 22°C (±2) ambient, and a reverse bias of 12 volts, unless otherwise stated. Shunt resistance measured at ± 10mV.

Absolute maximum ratings

Max. rating

dc reverse voltage 15V

Peak pulse current

(1µs, 1% duty cycle) 200mA

Peak dc current 10mA

Storage temperature range -45°C to +100°C Operating temperature range -25°C to +75°C Soldering temperature for 5 seconds

max. 200°C

Single elements

Active area 15mm² 3.8 ´ 3.8mm

Min. Typ. Max.

Responsivity A/W

l = 436nm 0.18 0.21

Dark current 3nA 10nA

NEP WHz l = 436nm

Vr = 0V 8.8 ´ 10^-14

Capacitance pF Vr = 0V 390

Capacitance pF Vr = 12V 80

Shunt resistance megaohm 25 200 Risetime ns l = 820nm

R_L= 50½ 12

Pin connections and dimensions

Figure 30 Typical spectral response

Medium area photodiode

(RS stock no. 651-995)

The RS stock no. 651-995 is a high speed, medium area, silicon photodiode mounted in an hermetically sealed TO5 package. The device is ideal for reduced light applications including brightness control, edge detectors, colour grading etc.

Absolute maximum ratings

Reverse voltage _____________________________60V Operating temperature range _______-40°C to +70°C Storage temperature range_________-55°C to +125°C Lead temperature soldering (5s)____________+200°C

Electrical characteristics

Parameter Conditions Min. Typ. Max. Unit

Radiant sensitive area 41.3 mm²

Wavelength of maximum sensitivity 760 800 880 nm

Peak responsivity 800 nm 0.4 0.5 A/W

Dark current V_R= 1V 4.0 20 nA

V_R= 20V 40 200 nA

Capacitance V_R= 0V 325 400 pF

V_R= 10V 91.5 113 pF

V_R= 20V 71 87.5 pF

Response time V_R= 10V, R_L= 100R 25 40 ns

Temperature coefficient of responsivity (0°C to +70°C) 0.35 %/°C

Temperature coefficient of dark current (0°C to +70°C) ´2 per +10°C

Typical performance curves

Figure 31 Normalised capacitance vs bias voltage

Figure 32 Open circuit voltage vs irradiation

Figure 33 Relative spectral response

Large area photodiode

(RS stock no. 303-674)

A high speed, large area, silicon photovoltaic detector housed in a 26.2mm diameter case. Its large active area, 1cm², and peak spectral response at 900nm make the device suitable for use as a calibration device in optical instrumentation, and for other optical measurements. Spectral response range (5% points):

350 to 1150nm.

Absolute maximum ratings

at +25°C (unless stated)

Reverse voltage V_R___________________________50V Operating temperature range _______-55°C to +70°C Forward current I_F____Limited by Pd and bias voltage Power dissipation Pd ______________________100mW

Features

● Photovoltaic operation or low-bias photoconductive operation

● High sensitivity over wide spectral range

● Circular active area (1cm²)

● Low noise

● Fast response

● Long term stability

● Low capacitance for a photovoltaic detector.

Applications

▲ Optical instrumentation

▲ Laser detection

▲ Optical communication.

Pin connections and dimensions

Electrical characteristics

Parameter Test conditions Min. Typ. Max. Unit

V_(BR) Breakdown voltage I_D= 100µA 50 V

I_D Dark current Dark, rev. bias 10V 0.5 1.5 µA

R_e Responsivity @ 450nm 0.2 0.22

633nm 0.35 0.4 A/W

900nm 0.5 0.55

1064nm 0.15 0.16

C Capacitance at 0V 1500 pF

at 10V rev. bias. 350

at 0V R_L50½ 0.5 µs

tI Response time (10% to 90%) at 10V R_L50½, l<910nm 50 ns

R_S Shunt resistance at 0V ±0.1V 5 M½

at 0V f = 1kHz 0.1 pA (rms)

In Noise current at 10V rev. bias f = 1kHz 0.4 Ã^Hz

*Refers to distance between window and active area

Typical performance curves

Figure 34 Typical spectral response.

Responsivity: 0.2A/W at 450nm, 0.35A/W at 633nm,

0.5A/W at 900nm, 0.15A/W at 1064nm, 7.9mA/Im (2850K source)

5mm

photodiode with amplifier

(RS stock no. 308-067)

The RS stock no. 308-067 consists of a high performance silicon photodiode combined with a high gain low noise amplifier in a TO5 package. It is designed particularly for use where accurate measurements are needed of low light levels, and medium speed variation in such light levels. Its small size and excellent temperature coefficients make it ideally suited for use under adverse conditions.

Any supply voltage between ±2.5V and ±18V may be used. A single output line gives a voltage with respect to earth (Pin 1) proportional to the input light level, up to a maximum only slightly less than the power rail. Correction for dark level output is not normally required due to its extremely low value.

The output may be short circuited to ground or either power rail without risk of damage. Changes in ambient temperature also cause only minimal variation in signal level, typically 150µV/°C.

Absolute maximum ratings

Supply voltage _____________________________±18V Output short circuit duration ______________Indefinite Storage temperature ______________-65°C to +100°C Operating temperature _______________0°C to +70°C

Connecting details

1. Earth 2. Output 3. V+

4. V- (Connected to can)

Features

● Very high responsivity

● Linear response

● Low output impedance

● Low noise

● Rugged construction

● Excellent temperature characteristics

● Short circuit proof

● Excellent power supply noise rejection

● TTL compatible

● Simple to use

Applications include

● Light intensity measurements

● Light fluctuation detection

● Optical spectroscopy

● Pollution monitoring

● Alarm systems

● Optical shaft encoders

● Automated inspection and control

● Flow monitoring.

CONNECTION DETAILS

Figure 35 Spectral and frequency response

Electrical specification

Parameter Conditions Min. Typ. Max. Units

O/P dark level +20 +60 mV

O/P saturation level R_L³2k½ -9 -12 V

O/P resistance¹ 75 ½

O/P short circuit current 6 mA

O/P noise voltage V_O= ²1V 1 3 mV/rms

Responsivity 430nm 30

630nm 160 mV/µW^-1cm²

900nm 250

Supply voltage (V+) 2.5 15 18 V

Supply voltage (V-) -2.5 -15 -18 V

Supply current R_L= ° 0.5 1.3 mA

Supply voltage rejection ratio 150 50 µV/V

Bandwidth Upper 3dB point 3 5 kHz

Rise time² C_L= 0 30 50 µs

Fall time² C_L= 0 30 50 µs

Dark level temperature coefficient 20°C ²TA²50°C 150 500 µV/°C

Notes:

1. At 5kHz. Drops to 0.01 at dc.

2. Time for output signal to reach 90% of true reading after application of a step change in light intensity.

Application examples

In Figure 36, the comparator (eg. RS stock no. 308- 843) will switch its output state when the light intensity increases above a pre-set level, determined by R_V. The centre zero voltmeter registers the difference between the switching threshold intensity and the actual intensity received by the RS stock no. 308-067.

Since the threshold is determined with respect to pin 1 of the RS stock no. 308-067 supply voltage variations have no effect on the operation of the circuit.

Note: Centre zero voltmeter (RS stock no. 259-628) requires a series resistor (RS stock no. 167-967).

Large area photodiode + amp

(RS stock no. 590-963)

This silicon photodiode has an active area of 100mm² and an integral transimpedance amplifier. This device is ideal for use in electrically noisy environments because the length of the highly sensitive input line to the amplifier is very short and is also screened by the metal can package.

Figure 37 TTL compatibility Figure 36 Light activated switch with

‘error’ monitor

Pin connection and case dimensions

The transimpedance configuration provides high noise immunity and high amplifier signal saturation levels.

Figure 38 Transimpedance amplifier circuit

Electrical characteristics (Ta = 25°C, ±15V supply)

In this circuit the op amp is used with negative feedback so that the current generated by the photo- diode is converted via the resistor into an output voltage.

Gain is defined only in terms of the feedback resistor.

UV enhanced photodiodes

(RS stock nos. 564-021, 564-037 and 564-043)

A range of ultra-violet sensitive silicon photodiodes with enhanced responsivity in the 190 to 400nm range.

The devices high shunt resistance and enhanced responsivity make them ideal for light measurement photometry and fluorescence applications.

The 5.8mm²is housed in a metal can package while the 33.6 and 100mm² devices are housed in ceramic packages. All packages incorporate a quartz window for enhanced spectral response.

Parameter Min. Typ. Max. Units

Operating wavelength 400 - 950 nm

Peak wavelength - 800 - nm

Responsivity @ 530nm 1.3 ´ 10⁵ - - V/W

Supply voltage ±5 - ±18 V

Supply current - - 250 µA

Transimpedance gain - 500k - -

Output resistance - 1 - ½

Dark output offset voltage - - ±5 mV

Rise time 77 - - µs

Bandwidth - 5 - kHz

Dark output noise level - - 400 µVrms

Temperature range 0° - +70 °C

Solder temperature 300°C for 15 seconds.

To eliminate unwanted oscillation it is recommended that a 10nF or 100nF disc ceramic capacitor in parallel with a 1µF tantalum decoupling capacitor be used between supply and 0V close to the device.

Absolute maximum ratings (Ta = 25°C)

Reverse voltage ______________________________5V Peak current (1µs, 1% duty cycle) ___________200mA Peak dc current ____________________________10mA Storage temperature range_________-55°C to +125°C Operating temperature range _______-55°C to +70°C Soldering temperature _____________________260°C Pin connections and case dimensions

RS stock no. 564-021

ATTENTION

OBSERVE PRECAUTIONS FOR HANDLING ELECTROSTATIC

SENSITIVE DEVICE

RS stock no. 564-037 RS stock no. 564-043

Electrical characteristics (Ta = 25°C)

Active Responsivity Dark current Rise time

area a/w (typical) Peak @Vr = Noise equivalent Capaci- V_L= 0½ Shunt resistance Responsivity 10mV power (typ.) tance R_L= 1k½ @Vr = ± 10mV mm² mm @ 190nm @ 245nm @ 340nm (typical) (typical) @900nm typical typical min. typical

5.8 2.4 ´ 2.4 0.12 0.14 0.19 950nm 3pA 6 ´ 10^-15w/Hz^1/2 170pF 0.4µs 0.5G½ 3G½ 33.6 5.8 ´ 5.8 0.12 0.14 0.19 950nm 20pA 1.5 ´ 10^-14w/Hz^1/2 1000pF 2.0µs 0.5G½ 0.5G½

Figure 39 Typical spectral response and typical quantum efficiency curves

Typical applications

Photovoltaic mode

In the photovoltaic mode, as the light level increases, photocurrent induced in the device develops a voltage across the dynamic shunt resistance. However, this resistance then decreases exponentially, therefore the photogenerated voltage is a logarithmic function of the incident light intensity.

Typically this mode of operation is most useful in simple comparator applications, Figure 40 shows the basic photovoltaic circuit.

Figure 40 Basic photovoltaic circuit

Photoamperic mode

If the photodiode is connected to a low value of load resistance the effect on the dynamic resistance is negligible and the output current is linearly related to light level.

The usual method of providing a low load resistance with subsequent amplification is to connect the diode to the virtual earth of an operational amplifier. This circuit as shown in Figure 41 is a current to voltage converter.

Photoconductive mode

The dynamic resistance of a reverse biased photodiode is constant, and a high value of load resistance can be used to give a voltage output that is linearly related to the light level incident on the device. Because the diode junction capacitance decreases with increasing reverse bias voltage, diodes operated in this mode will have the fastest response times. However, noise levels will also increase in this mode as leakage current increases with bias voltage. Figure 42 shows the basic circuit.

Figure 41 Photoamperic mode

Figure 42 Photoconductive mode

Linear photometer

Figure 43 illustrates a photometer circuit using an FET operational amplifier RS stock no. 307-058. Diode current I_Dvaries with light level from 1nA to 1mA type.

Resistor R is chosen to give required output-typ. value 1M½.

Figure 43 Linear photometer

16 element linear array

(RS stock no. 194-060)

A 16-element linear silicon PIN photodiode array housed in an hermetically sealed 24 pin ceramic d.i.l.

package. This high speed device consists of 16 indi- vidual elements arranged on a 1mm pitch in common cathode configuration. This array is ideal for linear position sensing, wide aperture detection and edge and hole detection in strip materials.

Peak

responsivity Dark current Capacitance Response time

per diode Number Pitch Active area per diode per diode per diode

Package A/W at l of of diodes of each diode nA pF nS

type nm diodes mm mm² (VR = 1V) (VR = 0V) (VR = 10V RL = 100R)

24 pin 0.6 900 16 1.0 0.66 0.1 9 4

Figure 44 Spectral response Figure 46 Open circuit voltage vs irradiation

Figure 45 Normalised capacitance vs reverse bias